When is it ethical to pay clinical trial participants different amounts?

This collaboration grew out of our shared experience as scholars studying research ethics and (for some of us) as institutional review board members. We witnessed debates over when, if ever, it can be fair to pay different participants in the same study different amounts of money. Situations where an investigator might want to provide differential incentives can arise in research. For instance, the second wave of a clinical trial might require more money to incentivize recruitment than the first wave did. Or subjects recruited at one site of a multi-site study may move to a different site with different payment levels. Some of these debates have led to regulatory changes—for instance, the South African Medicines Control Council adopted a proposal that all trials be required to pay the same amount of money.

Building on work two of us (Emily and Holly) had already done, we divided differential payment into three categories: reimbursement for costs incurred, compensation for time, burden, or contribution, and incentives to participate. A key premise of our argument is that differential payment can help promote the scientific and ethical goals of clinical research. We argue that differential reimbursement and compensation should be ethical defaults when participants incur different costs or burdens to treat them fairly. We also argue that differential incentives can be ethical if needed to advance the scientific ends of a study, but recognize that incentives are the most ethically controversial of the three categories.

Ethical controversy around differential incentives is not unique to incentives in research. In 1986, Kahneman, Knetsch, and Thaler observed that 73% of people viewed it as acceptable to pay a later-hired worker a different amount than an earlier-hired one, if background economic circumstances had changed since the first worker was hired:

A small photocopying shop has one employee who has worked in the shop for six months and earns $9 per hour. Business continues to be satisfactory, but a factory in the area has closed and unemployment has increased. Other small shops have now hired reliable workers at $7 an hour to perform jobs similar to those done by the photocopy shop employee. The current employee leaves, and the owner decides to pay a replacement $7 an hour.

But Kahneman, Knetsch, and Thaler also found that people’s judgments about the ethics of differential incentives are highly context-dependent. In the same study, 64% of people judged it unacceptable to choose between four qualified applicants for a job by selecting the one requesting the lowest salary.

Even though we argue that differential payment is ethically acceptable, we recognize that it could potentially create tension among clinical trial participants. For instance, with trial participants’ increasing use of social media, it is now easier for participants to find one another and to discuss how much they are being paid for participation. These discussions could lead to participants becoming upset if they believe they are unjustifiedly being paid less than others. Examining prospective participants’ attitudes about the acceptability of differential payment would be a fruitful area for future empirical research.